Polarization diversity detection without a polarizing beam splitter

ABSTRACT

A fiber optic measurement device including an optical frequency domain reflectometer ( 10 ) performs polarization diversity detection without using a polarizing beam splitter.

CLAIM OF BENEFIT OF PROVISIONAL PATENT APPLICATION

Priority is claimed from U.S. Provisional Patent Application No.60/394,260, filed on Jul. 9, 2002. The contents of this provisionalapplication are incorporated by reference.

RELATED APPLICATIONS

This application is related to commonly-assigned PCT Application No.______, entitled “Heterodyne Optical Spectrum Analyzer,” filed on Jul.8, 2003, and to commonly-assigned, U.S. patent application Ser. No.10/005,819, entitled “Apparatus and Method for the CompleteCharacterization of Optical Devices Including Loss, Birefringence, andDispersion Effects,” filed on Dec. 14, 2001.

FIELD OF THE INVENTION

The present invention relates to optical measurements, and moreparticularly, to a device and method for performing polarizationdiversity detection.

BACKGROUND AND SUMMARY OF THE INVENTION

Mixing between a reference signal and a data signal is often necessaryto extract information about an optical device. A probe signal and areference signal originating from the same source are typically mixed,resulting in fringes that can be detected and used to asses informationabout the device being probed. In interferometric sensing, a referencesignal is mixed with a signal whose phase and/or amplitude is modifiedby a parameter to be measured.

The mixing produces an interference signal, and the amplitude of theinterference signal depends on how efficiently the two optical signalsmix. When the two signals have the same polarization state, the mixingefficiency is 100%. When the two signals have orthogonal polarizationstates, no mixing occurs—0% efficiency. Between these two limits, onlythe portion of the signals whose polarization states resolve onto asingle polarization axis actually mix. The reduced, mixed-signalamplitude results from an unmixed component in an orthogonalpolarization state. This inefficiency is usually referred to aspolarization induced fringe fading.

Polarization diversity detection overcomes polarization induced fading.One commonly known interferometric scheme that can suffer frompolarization fading is Optical Frequency Domain Reflectometry (OFDR).OFDR injects a highly monochromatic beam of light into the opticalsystem or device to be tested. The frequency of that light is variedslowly with a time-linear sweep, and the optical signal back-scatteredfrom the optical system is detected by coherently mixing theback-scattered signal with the reference input signal. The beatfrequency component of the mixed signal, (corresponding to aninterference signal), is measured to determine a position of theback-scattering (reflection) point in the optical system/fiber. Theinterference signal amplitude also determines a back-scattering factorand an attenuation factor for the reflected light.

U.S. Pat. Nos. 6,376,830 and 5,789,521 provide further details regardingOFDR measurement and are incorporated herein by reference. Reference mayalso be made to commonly-assigned, U.S. patent application Ser. No.10/005,819, entitled “Apparatus and Method for the CompleteCharacterization of Optical Devices Including Loss, Birefringence, andDispersion Effects,” filed on Dec. 14, 2001.

A single mode optical fiber supports two degenerate polarization modes.This degeneracy causes field energy to be transferred between the modesas they propagate down the fiber. This phenomenon causes thepolarization fading in fiber-optic interferometers. FIG. 1 showsschematically a Mach-Zender interferometer. The arrows denote electricfield (E) vector components. Polarization fading occurs whenever E₁ andE₂ are not co-linear, i.e., {overscore (E)}₁·{overscore(E)}₂=|{overscore (E)}₁∥{overscore (E)}₂|cos θ, θ≠0. The power measuredat the detector is proportional to the square of the absolute value of(E₁+E₂). The interference terms of this relationship are proportional toE₁·E₂*+E₂·E₁*, where * denotes a complex conjugate. When a first couplerC1 splits the input field E_(in), there is a chance that the splitfields E₁ and E₂ in the respective interferometer arms evolve intoorthogonal polarizations. As described above, in that situation, nointerference fringes will be detected, and there is completepolarization fading or 0% mixing efficiency.

A worst case scenario in which the fields interfering on the detector,E₁ and E₂, are orthogonal is shown in FIG. 2. More formally, in someorthogonal basis, the fields can be written E1=(a, 0)exp(iωτ) and E₂=(0,d), where τ is the propagation time difference between the twointerferometer arms (τ=n_(e)L/c, where n_(e) is the effective (modal)index of the fiber. The basis of a vector set includes two vectors intwo dimensions or three vectors in three dimensions that are used torepresent all other possible vectors. Knowing the basis of a vector setis essentially the same as knowing the coordinate system for a point inspace. For example, a location may be described as being at 32 degreesNorth and 25 degrees West. The coordinate system is the set of latitudeand longitude lines on the Earth, and the particular location isunderstood. The basis set is a pair of vectors, each one degree (60nautical miles) long, with one vector pointed to the North and onevector pointed to the West.

Now in the S-P basis set, shown in FIG. 2 as orthogonal, the fields canbe written as E₁=(a′, b′)exp(iωτ) and E₂=(c′, d′) so E₁·E₂=0, butE₁+E₂=(a′ exp (iωτ)+c′, b′ exp (i ωτ)+d′). Polarization diversitydetection detects the s and p components (or projections onto the s andp axes) of E₁+E₂ separately using two S and P detectors. The power ateach detector is proportional to the modulus squared of the componentsof the total field:P _(S) ∝|a′ exp(iωτ)+c′| ²  (1)P _(P) ∝|b′ exp(iωτ)+d′| ²  (2)These diversity power signals exhibit fringes even though the totalfield, i.e., the sum of two orthogonal fields, does not.

Polarization diversity detection may be implemented using a polarizingbeam splitter (PBS) as show in FIG. 3. If the field at the PBS is E_(bS)and is given by E_(bs)=(A, B) in the basis set of the polarizing beamsplitter, then the measured powers at the S and P detectors areP_(S)∝|A|² and P_(p)∝|B|². When the PBS splits the field into differentcomponents, the crystalline structure of the PBS imposes an orthonormalbasis onto which the incident field is projected. That orthonormal basisis needed to extract information contained in the E₁ and E₂ amplitudes.

But there are drawbacks with using polarizing beam splitters. First,they are bulky and expensive. Second, polarizing beam splitters addstray reflections to the detected signals. Third, if the polarizing beamsplitter is designed to operate in a particular wavelength, e.g., 1500nm, it cannot be easily and inexpensively altered to operate at anon-standard wavelength, such as 800 nm, at least as compared to astandard optical coupler. For these and other reasons, it is an objectof the present invention to perform polarization diversity detectionwithout a polarizing beam splitter.

The present invention performs polarization diversity detection withoutusing a polarizing beam splitter. Field vectors from one interferometerarm are used as the basis upon which to project a field vector from theother interferometer arm. Polarization diversity detection is performedusing only standard optical couplers, e.g., 50-50 couplers. Apolarization beam splitter is not needed.

A first coupler receives a first optical signal from a device or systemunder test and generates first and second coupler outputs. A secondcoupler receives a second optical signal from a reference source andgenerates third and fourth coupler outputs. A first polarizationcontroller (PC) changes the polarization state of the third coupleroutput and generates a PC output. A third coupler generates a firstcombined output from the first coupler output and the PC output. Afourth coupler generates a second combined output from the secondcoupler output and the fourth coupler output. A first detector detects afirst power of the first combined output in a first projection plane,and a second detector detects a second power of the second combinedoutput in a second projection plane. A processor processes interferenceterms in the first and second powers in the first and second projectionplanes to determine one or more characteristics of the first opticalsignal.

A second polarization controller changes the polarization of the firstoptical signal before it is received in the first optical coupler. Thefirst and second polarization controllers are adjusted to calibrate thefiber optic measurement device. Different second polarization controllersettings result in multiple corresponding vector measurements at thefirst and second detectors. The processor calculates a vectorcalibration matrix using these vector measurements. The processorcorrects subsequent detected vector measurements using the vectorcalibration matrix. The corrected vector measurements ensure that thevector representation of the first optical signal are in an ortho-normalbasis set.

The OFDR components can be constructed simply using optical fiber, andif desired, from the same type of standard low-loss fiber. Matching thetype of fiber throughout the optical network results in very low losseswith essentially zero scattering events in the network. As a result, theOFDR produces very clean time domain measurements (only reflectionevents from the device under test appear).

Another advantage of fiber-based OFDR construction is significant costreduction and increased reliability and flexibility. A polarizationcontroller can be implemented simply as a single loop of fiber that ismoved to achieve a certain polarization state at the output. Once theloop is positioned, it need not be moved again. Couplers are constructedby melting two optical fibers together. In order to manufacture couplersfor operation at widely different wavelengths, (e.g., 615 nm and 1550nm), coupler manufacturers need only purchase fiber (an inexpensivecommodity) designed for that wavelength and melt two sections togetherusing the same process for all wavelengths. No re-tooling or significantchanges to the process are required. As a result, couplers are readilyavailable at all wavelengths at a reasonable price in contrast topolarization beam splitters and other bulk-optic based opticalcomponents.

Other features, aspects, and advantages of the present invention willbecome apparent from the following detailed description, taken inconjunction with the accompanying drawings, illustrated by way ofexample the principles of the invention. Like reference symbols refer tolike elements throughout.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a Mach-Zender interferometer;

FIG. 2 illustrates orthogonal measurement field vectors E₁ and E₂ andtwo basis vectors S and P;

FIG. 3 illustrates a Mach-Zender interferometer with a polarization beamsplitter;

FIG. 4 illustrates in function block format an optical frequency domainreflectometer (OFDR) for polarization diversity detection without apolarization beam splitter;

FIG. 5 illustrates a different configuration of the OFDR shown in FIG.4;

FIG. 6 illustrates in further detail the detectors shown in FIGS. 4 and5;

FIG. 7 illustrates in further detail the data acquisition block in FIGS.4 and 5;

FIG. 8 is a vector diagram showing the measurement field vector E₁ andreference field vector E₂ each being projected and summed on each of thebasis axes S and P in accordance with projections implemented by apolarization beam splitter;

FIG. 9 is a similar vector diagram for the all coupler OFDRimplementations found in FIGS. 4 and 5; and

FIG. 10 is a vector projector diagram showing E₁ projected onto virtualreference fields E′_(S) and E′_(P).

DETAILED DESCRIPTION OF THE INVENTION

The following description, for purposes of explanation not limitation,sets forth specific details, such as particular components, electroniccircuitry, techniques, etc. in order to provide an understanding of thepresent invention. But it will be apparent to one skilled in the artthat the present invention may be practiced in other embodiments thatdepart from these specific details. In other instances, detaileddescriptions of well-known methods, devices, and techniques, etc. areomitted so as not to obscure the description with unnecessary detail.Individual function blocks are shown in the figures. Those skilled inthe art will appreciate that functions may be implemented using discretecomponents or multi-function hardware. Processing functions may beimplemented using a programmed microprocessor or general-purposecomputer, using an application specific integrated circuit (ASIC),and/or using one or more digital signal processors (DSPs).

A first, non-limiting, example OFDR embodiment that does not employ apolarizing beam splitter is described in conjunction with FIG. 4. AnOFDR 10 includes a tunable laser 12 for generating an electric field ata particular frequency (controlled by the frequency sweep signal fromprocessor 32) provided to a standard optical coupler 14. Any suchcoupler may be employed, and one non-limiting example is Gould partnumber 23-40355-33-01201 manufactured by Gould Fiber Optics Division ofGould Electronics of Baltimore, Md. Coupler 14 splits the input fieldE_(IN) into two electric field signals E₁ and E₂. E₁ is provided throughoptical coupler 36 and connector 38 to a device or system under test(DUT) 40. A back-scattered signal E₁ to be measured as a function of itsreflection point along the fiber is provided through coupler 36 to afirst coupler 16.

The reference signal E₂ is provided to a second coupler 22. Apolarization state of a first output of coupler 22 is changed inpolarization controller 24. The output of polarization controller 24 isthe reference signal E₂ in a first reference plane denoted “S” so thatthis reference signal is referred to as E_(S). The second output ofcoupler 22 corresponds to the reference signal in another referenceplane labeled “P” so that this signal is denoted E_(P). The first outputof coupler 16 is E_(X) and equals M₁₃E₁ as described below. The secondoutput of coupler 16 is E_(Y) and equals M₁₄E₁.

The couplers 18 and 26 output the signals E_(X)+E_(S) and E_(Y)+E_(P),respectively, which are detected by respective detectors 20 and 28. Theoutput of S-detector 20 is a power P_(S), and the output of P-detector28 is a power P_(P). Both powers are provided to a data acquisition unit30 which provides digital information to processor 32. The processor 32processes the information and generates the desired electric fieldoutput signal E_(OUT) which is then provided to a display 34 to displayone or more parameters of E_(OUT). Such parameters may include amplitudeand phase of the scattered light and the particular location at whichthe reflection occurs. Processor 32 sweeps the tunable laser 12 througha particular sweep range specified by a starting wavelength and afinishing wavelength, e.g., 1540 nm-1580 nm.

FIG. 5 illustrates another example embodiment with a slightly differentconfiguration in which the device under test 40 is coupled directly tothe output of the coupler 14. Both embodiments employ a polarizationcontroller 42 used in calibrating the OFDR 10 as will be laterdescribed.

The detectors 20 and 28 are illustrated in further detail in FIG. 6. Anysuitable detector may be employed, and one non-limiting example is aThorLabs PDA 400 optical detector manufactured by ThorLabs of Newton,N.J. Each detector includes a photodetector 42 and an amplifier 44coupled to a low-pass filter 46. The data acquisition block 30 includesan analog-to-digital conversion block 48 coupled to a buffer 50. Thefiltered output from the detector is converted into a digital format bythe digital-to-analog conversion means 48, and the digital signal isstored in the buffer 50 before being pr processed by the data processor32.

The vector diagram in FIG. 8 shows projected fields on the S and P powerdetector reference planes. The reference fields S and P are assumedorthogonal—a reasonable assumption if a PBS is used. The S component orprojection of the measured field E₁ is denoted E_(X) on the horizontalaxis, and the P component or projection of the measured field E₁ isdenoted E_(Y) along the vertical axis. The reference field E₂ is alsoprojected onto the S and P axes. The sum of E_(X) and E_(S) is detectedon the S detector 20, and the sum of the projections E_(P) and E_(Y) isdetected on the P detector 28.

But when the two fields E₁ and E₂ are detected by the coupler pair 16and 22, the S and P axes cannot be assumed to be orthogonal or even thesame length. Although the interference takes place at two separatedetectors between signals traveling significantly different paths, thatinterference can be represented as the projection of the original signalof interest E1 onto two non-parallel vectors. To account for thenon-orthonormal basis, E₁ is altered by two transforming matrices M₁₃and M₁₄ prior to being projected onto the reference fields E_(S) andE_(P) as shown in FIG. 9. So long as the two transforming matrixes M₁₃and M₁₄ do not vary with time, this is an acceptable transformation.

Rather than the projection of E₁ onto the S and P axes, FIG. 9 shows theprojection of E_(X) onto E_(S) and the projection of E_(Y) onto E_(P).Even though the transforming matrices M₁₃ and M₁₄ are unknown, thereference fields E_(S) and E_(P) may still be transformed in a preciseway that allows the detected fields as projections of E₁ onto some setof vectors. This is illustrated in FIG. 10 in which E₁ is projected ontotwo non-parallel vectors E′_(S)=M⁻¹ ₁₃ E_(S) and E′_(P)=M⁻¹ ₁₄ E_(S). Aswill be demonstrated below, E₁ can be recovered from these projectionsshown in FIG. 10 using a linear mathematical transformation.

The propagation of a field in an optical fiber from one location toanother through any linear section of the system (e.g., optical fiber,optical component, etc.) can be represented by a complex 2×2 matrix.This matrix will account for all effects of the linear section includingloss, polarization rotation, and polarization-dependent loss. Let thepropagation from coupler i to coupler j (i, j=1,2,3,4) be represented bythe matrix M_(ij). We therefore have {overscore (E)}_(x)=M₁₃{overscore(E)}₁ and {overscore (E)}_(y)=M₁₄{overscore (E)}₁. The interferenceterms measured at the S- and P-detectors 20 and 28 are proportional toP _(s) ∝{overscore (E)} _(x) ·{overscore (s)}*+{overscore (E)} _(x)*·{overscore (s)}=M ₁₃ {overscore (E)} ₁ ·{overscore (s)}*+(M ₁₃{overscore (E)} ₁)*·{overscore (s)},  (3)P _(p) ∝{overscore (E)} _(y) ·{overscore (p)}*+{overscore (E)}* _(y)·{overscore (p)}=M ₁₄ {overscore (E)} ₁ ·{overscore (p)}*+(M ₁₄{overscore (E)} ₁)*·{overscore (p)}.  (4)

As described, without a polarizing beam splitter, the vectors S and P nolonger form an orthonormnal basis. But knowledge of the amplitude andrelative angle between the vectors S and P allows the reconstruction ofE₁ in an orthogonal basis.

From Eqs. (3) and (4), it is seen that the detector power measurementsof P_(s) and P_(p) project the vectors M₁₃E₁ and M₁₄E₁ into the S-Pbasis. The fact that the basis-vectors S and P are arbitrary allows useof the identity, {overscore (x)}·(M{overscore (y)})={overscore(y)}·(M^(t){overscore (x)}), where x and y are arbitrary vectors, M isan arbitrary matrix, and M^(t) is the transpose of matrix M, to writethe following:(M ₁₃ {overscore (E)} ₁)·{overscore (s)}={overscore (E)} ₁·(M _(—) ^(t){overscore (s)}*)={overscore (E)} ₁ ·{overscore (s)}′  (5)(M ₁₄ {overscore (E)} ₁)·{overscore (p)}={overscore (E)} ₁·(M ₁₄ ^(t){overscore (p)}*)={overscore (E)} ₁ ·{overscore (p)}′  (6)

The vectors {overscore (s)}′ and {overscore (p)}′ act as the basisvectors onto which E₁ is projected. Knowledge of the amplitudes of andrelative angle between {overscore (s)}′ and {overscore (p)}′ allows theprojection of E₁ onto an orthogonal basis set. What is required is aprocess by which this correcting matrix can be quickly and efficientlyfound to transform the measurements into an ortho-normal basis set.

Power measurements at the S and P detectors yield information about thevector field {overscore (E)}={overscore (E)}_(x)+{overscore (E)}_(y) inthe S-P basis set. Those measurements are of the formP _(s) =|E _(x)|² +|s| ²+2E _(x) s cos φ_(x)  (7)P _(p) =|E _(y)|² +|p| ²+2E _(y) p cos φ_(y)  (8)

Omitting dc components, we can form the vector, {overscore (v)}=(2E_(x)scos φ_(x),2E_(y)p cos φ_(y))=(E_(s), E_(p)). But again E_(s) and E_(p)are not orthogonal. To remedy this, a calibration matrix, M, isdetermined. When it is multiplied by v, the product gives a new vector Ethat represents the field E₁ in a calibrated, orthogonal basis.

The calibration begins by adjusting the polarization controllers PC₁ andPC₂ (41 and 24). With the reference laser 12 in the continuous sweepmode, PC₁ is adjusted so that the fringes observed on the P-detector 28are maximized. When this is accomplished, the fringes on the S-detector20 are minimized by adjusting PC₁. When this is accomplished, PC₁ isadjusted so the fringe levels on the S- and P-detectors areapproximately equal (to within ±10%).

Once the polarization controllers PC₁ and PC₂ are adjusted, the OFDR canbe calibrated by taking measurements of {overscore (v)}=(2E_(x)s cosφ_(x),2E_(y)p cos φ_(y)) for four distinct but random settings of PC₁.The following represent these measurements: $\begin{matrix}\begin{matrix}\begin{matrix}{{{\overset{\rightharpoonup}{v}}_{1} = \begin{pmatrix}E_{s^{1}} \\E_{p^{1}}\end{pmatrix}},} & {{{\overset{\rightharpoonup}{v}}_{2} = \begin{pmatrix}E_{s^{2}} \\E_{p^{2}}\end{pmatrix}},}\end{matrix} \\\begin{matrix}{{{\overset{\rightharpoonup}{v}}_{3} = \begin{pmatrix}E_{s^{3}} \\E_{p^{3}}\end{pmatrix}},} & {{\overset{\rightharpoonup}{v}}_{4} = {\begin{pmatrix}E_{s^{4}} \\E_{p^{4}}\end{pmatrix}.}}\end{matrix}\end{matrix} & (9)\end{matrix}$

With the above definitions, the following matrix can be formed$\begin{matrix}{\begin{bmatrix}p & g \\q & h\end{bmatrix} = {\begin{bmatrix}{\overset{\rightharpoonup}{v}}_{1} & {\overset{\rightharpoonup}{v}}_{2}\end{bmatrix}^{- 1}\begin{bmatrix}{\overset{\rightharpoonup}{v}}_{3} & {\overset{\rightharpoonup}{v}}_{4}\end{bmatrix}}} & (10)\end{matrix}$where [x y] is a matrix with columns formed by the elements of thevectors x and y. Using the following set of definitions:A = 1 − p² − q² B = 1 − g² − h² C = 2  Re⌊p^(*)q⌋D = −2  Im⌊p^(*)q⌋ E = 2  Re⌊g^(*)h⌋ F = −2  Im⌊g^(*)h⌋ $\begin{pmatrix}x \\y\end{pmatrix} = {\begin{pmatrix}C & D \\E & F\end{pmatrix}^{- 1}\begin{pmatrix}A \\B\end{pmatrix}}$ α = x + 𝕚  y $\beta = \sqrt{1 - {\alpha }^{2}}$$\hat{M} = {\begin{pmatrix}1 & \alpha \\0 & \beta\end{pmatrix}\begin{bmatrix}{\overset{\rightharpoonup}{v}}_{1} & {\overset{\rightharpoonup}{v}}_{2}\end{bmatrix}}^{- 1}$the vector-calibration matrix is given by $\hat{M} = {\begin{pmatrix}1 & \alpha \\0 & \beta\end{pmatrix}\begin{bmatrix}{\overset{\rightharpoonup}{v}}_{1} & {\overset{\rightharpoonup}{v}}_{2}\end{bmatrix}}^{- 1}$Any measurement vector {overscore (v)}_(m)=(2E_(mx)s cos φ_(mx),2E_(my)pcos φ_(my)) can be corrected by performing the following multiplication{overscore (E)}={circumflex over (M)}{overscore (v)}_(m)where, after the above multiplication, E is guaranteed to be in someorthonormal basis.

Although the above-description is directed to the two polarization modesof standard optical fiber, optical fiber can support a variety ofdifferent modes. To handle that mode variety, one coupler and onedetector would be added for each new mode present in the fiber. “ModeControllers” corresponding to fiber loops (like the polarizationcontroller loops) would also be used in each reference path. Calibrationwould be carried out using analogous linear algebra operations. Theabsence of stray reflections as described above means that the inventionis particularly effective at measuring the very low scatter levels thatcome from the non-homogeneities in the optical fiber core.Optical-fiber, scatter-level measurements can be used to measure losseswithin an optical network independently of the manner of connection tothe network.

While the invention has been described in connection with practical andpreferred embodiments, the invention is not limited to the disclosedembodiments. On the contrary, the invention covers various modificationsand equivalent arrangements included within the scope of the appendedclaims.

1. A fiber optic measurement device comprising an optical frequencydomain reflectometer (OFDR) configured to employ polarization diversitydetection without using a polarizing beam splitter.
 2. The fiber opticmeasurement device according to claim 1, further comprising: a firstcoupler for receiving a first optical signal from a device or systemunder test and generating first and second coupler outputs, and a secondcoupler for receiving a second optical signal from a reference sourceand generating third and fourth coupler outputs.
 3. The fiber opticmeasurement device according to claim 2, further comprising: apolarization controller (PC) for changing a polarization state of thethird coupler output and generating a PC output; a third coupler forreceiving the first coupler output and the PC output and generating afirst combined output; and a fourth coupler for receiving the secondcoupler output and the fourth coupler output and generating a secondcombined output.
 4. The fiber optic measurement device according toclaim 3, further comprising: a first detector for detecting a firstpower of the first combined output in a first projection plane, and asecond detector for detecting a second power of the second combinedoutput in a second projection plane.
 5. The fiber optic measurementdevice according to claim 4, further comprising: processing circuitryfor processing interference terms of the first and second powers in thefirst and second projection planes to determine one or morecharacteristics of the first optical signal.
 6. The fiber opticmeasurement device according to claim 5, wherein the fiber opticmeasurement device accounts for polarization of the first optical signalwithout using a polarizing beam splitter.
 7. The fiber optic measurementdevice according to claim 5, further comprising: a second polarizationcontroller for changing a polarization of the first optical signalbefore being received in the first optical coupler, wherein the firstand second polarization controllers are adjustable for calibrating thefiber optic measurement device, wherein for multiple different settingsof the second polarization controller resulting in multiplecorresponding vector measurements at the first and second detectors, theprocessing circuitry is configured to calculate a vector calibrationmatrix using the vector measurements.
 8. The fiber optic measurementdevice according to claim 7, wherein the processing circuitry isconfigured to correct detected vector measurements using the vectorcalibration matrix such that the corrected vector measurements result ina vector representation of the first optical signal in an orthonormalbasis.
 9. An optical frequency domain reflectometer (OFDR) configured toemploy polarization diversity detection comprising: a first coupler forreceiving a first optical signal from a device or system under test andgenerating first and second coupler outputs; a second coupler forreceiving a second optical signal from a reference source and generatingthird and fourth coupler outputs; a polarization controller (PC) forchanging a polarization state of the third coupler output and generatinga PC output; a third coupler for receiving the first coupler output andthe PC output and generating a first combined output; a fourth couplerfor receiving the second coupler output and the fourth coupler outputand generating a second combined output; a first detector for detectinga first power of the first combined output in a first projection plane;a second detector for detecting a second power of the second combinedoutput in a second projection plane; and processing circuitry forprocessing interference terms of the first and second powers in thefirst and second projection planes to determine one or morecharacteristics of the first optical signal.
 10. The OFDR according toclaim 9, wherein the OFDR accounts for polarization of the first opticalsignal without using a polarizing beam splitter.
 11. The OFDR accordingto claim 9, further comprising: a second polarization controller forchanging a polarization of the first optical signal before beingreceived in the first optical coupler, wherein the first and secondpolarization controllers are adjustable for calibrating the fiber opticmeasurement device, and wherein for multiple different settings of thesecond polarization controller resulting in multiple correspondingvector measurements at the first and second detectors, the processingcircuitry is configured to calculate a vector calibration matrix usingthe vector measurements.
 12. The OFDR according to claim 11, wherein theprocessing circuitry is configured to correct detected vectormeasurements using the vector calibration matrix such that the correctedvector measurements result in a vector representation of the firstoptical signal in an orthonormal basis.
 13. A method comprisingdetecting one or more parameters of an optical signal using polarizationdiversity detection without using a polarizing beam splitter.
 14. Themethod according to claim 13, further comprising: receiving at a firstcoupler a first optical signal from a device or system under test andgenerating first and second coupler outputs, and receiving at a secondcoupler a second optical signal from a reference source and generatingthird and fourth coupler outputs.
 15. The method according to claim 14,further comprising: changing in a first polarization controller apolarization state of the third coupler output and generating a changedthird coupler output; receiving at a third coupler the first coupleroutput and the changed third coupler output and generating a firstcombined output; and receiving at a fourth coupler the second coupleroutput and the fourth coupler output and generating a second combinedoutput.
 16. The method according to claim 15, further comprising:detecting a first power of the first combined output in a firstprojection plane, and detecting a second power of the second combinedoutput in a second projection plane.
 17. The method according to claim16, further comprising: processing interference terms of the first andsecond powers in the first and second projection planes to determine oneor more characteristics of the first optical signal.
 18. The methodaccording to claim 17, further comprising: changing in a secondpolarization controller a polarization of the first optical signalbefore being received in the first optical coupler; for multipledifferent settings of the second polarization controller, generatingmultiple corresponding detected vector measurements; calculating avector calibration matrix using the vector measurements.
 19. The methodaccording to claim 18, further comprising: correcting detected vectormeasurements using the vector calibration matrix such that the correctedvector measurements result in a vector representation of the firstoptical signal in an ortho-normal basis.